Impact additive of the core/shell type for thermoplastic polymers

ABSTRACT

The invention relates to an impact additive of the core/shell type for thermoplastic polymers.This impact additive comprises a crosslinked elastomeric core based on n-alkyl acrylate and a shell made of poly(alkyl methacrylate) grafted onto the said core.

BACKGROUND OF THE INVENTION

The invention relates to an impact additive of the core/shell type aswell as to a composition containing a thermoplastic polymer, inparticular a vinyl chloride homopolymer or a copolymer mostly containingvinyl chloride, an impact additive of the core/shell type and optionallyother additives.

Some synthetic resins, in particular resins based on poly (vinylchloride) or on a copolymer mostly containing vinyl chloride, are widelyused in the building industry, in particular due to their low cost andto their good physical and/or chemical properties.

Nevertheless, they exhibit low impact strengths at ambient temperatureor at low temperature or again also after ageing.

It has been proposed to overcome these defects by incorporating, inthese thermoplastic resins, products known as impact additives which aregenerally polymers exhibiting a degree of elastomeric properties.

A description is given in U.S. Pat. No. 3,678,133 of an impact additiveof the core/shell type composed of an elastomeric core and of a morerigid thermoplastic shell.

The elastomeric core is obtained by polymerization of a mixture ofmonomers comprising at least 50% by weight of an alkyl acrylate, thealkyl group of which has from 2 to 8 carbon atoms, and a minorproportion of a crosslinking agent. The preferred alkyl acrylate isn-butyl acrylate.

It is also mentioned that alkyl acrylates having longer chains exhibitthe disadvantage of polymerizing with greater difficulty, 2-ethylhexylacrylate being given as an example.

The rigid thermoplastic shell is obtained by polymerization of a mixtureof monomers comprising 40% to 100% by weight of alkyl methacrylate inwhich the alkyl group contains 1 to 4 carbon atoms.

The impact additive in this patent is produced in such a way that thepolymerization of the rigid thermoplastic shell takes place at thesurface of the elastomeric phase, preferably as a separate layer whichmore or less completely covers the elastomeric core.

Although the impact additives thus obtained significantly improve theimpact strength at ambient temperature of the resins containing them,there is a loss in the mechanical properties, in particular a loss inthe impact strength, at low temperature, of the said resins.

SUMMARY OF THE INVENTION

An impact additive of the core/shell type has now been found which iscomposed of a core based on alkyl acrylate or on a polyorganosiloxanerubber and a shell based on poly(alkyl methacrylate), or on astyrene-acrylonitrile copolymer, characterized in that the said impactadditive comprises from:

(a) 70% to 90% by weight, and preferably 75% to 85%, of an elastomericcrosslinked core which is composed:

1) of 20% to 100% by weight, and preferably of 20% to 90%, of a nucleuscomposed of a copolymer (I) of n-alkyl acrylate, the alkyl group ofwhich has a carbon number ranging from 5 to 12, and preferably rangingfrom 5 to 8, or of a mixture of alkyl acrylates, the linear or branchedalkyl group of which has a carbon number ranging from 2 to 12, andpreferably ranging from 4 to 8, or of a polyorganosiloxane rubber, of apolyfunctional crosslinking agent possessing unsaturated groups in itsmolecule, at least one of which is of CH₂═C< vinyl type, and optionallyof a polyfunctional grafting agent possessing unsaturated groups in itsmolecule, at least one of which is of CH₂═CH—CH₂—allyl type, the saidnucleus containing a molar amount of crosslinking agent and optionallyof grating agent ranging from 0.05% to 5% and preferably an amount ofbetween 0.5% and 1.5%;

2) of 80% to 0% by weight, and preferably of 80% to 10%, of a coveringcomposed of a copolymer (II) of n-alkyl acrylate, the alkyl group ofwhich has a carbon number ranging from 4 to 12, and preferably rangingfrom 4 to 8, or of a mixture of alkyl acrylates as defined above in 1)and of a polyfunctional grafting agent possessing unsaturated groups inits molecule, at least one of which is of CH₂═CH—CH₂—allyl type, thesaid covering containing a molar amount of grafting agent ranging from0.05% to 2.5% and preferably an amount of between 0.5% and 1.5%;

b) 30% to 10% by weight, and preferably 25% to 15%, of a shell graftedonto the said core composed of a polymer of an alkyl methacrylate, thealkyl group of which has a carbon number ranging from 1 to 4, oralternatively of a statistical copolymer of an alkyl methacrylate, thealkyl group of which has a carbon number ranging from 1 to 4, and of analkyl acrylate, the alkyl group of which has a carbon number rangingfrom 1 to 8, containing a molar amount of alkyl acrylate ranging from 5%to 40%, and preferably of between 10% and 20, or alternatively composedof a styrene-acrylonitrile copolymer having a preferredstyrene:acrylonitrile molar ratio between 1:1 and 4:1, and particularlybetween 7:3 and 3:1, respectively.

Mention will be made, as illustration of n-alkyl acrylates which can beused according to the present invention to form the copolymer (I), ofn-pentyl acrylate, n-hexyl acrylate, n-heptyl acrylate and veryparticularly n-octyl acrylate.

Mention will be made, as illustration of n-alkyl acrylates which can beused according to the present invention to form the copolymer (II), ofn-butyl acrylate, n-pentyl acrylate, n-hexyl acrylate, n-heptyl acrylateand very particularly n-octyl acrylate.

The n-alkyl acrylate which may be used to form the copolymers (I) and/or(II) can be identical or different.

Mention will be made, as illustration of linear or branched alkylacrylates which can be used according to the present invention for theformation of the mixtures of alkyl acrylates constituting the copolymers(I) and /or (II), of ethyl acrylate, n-propyl acrylate, n-butylacrylate, amyl acrylate, 2-methylbutyl acrylate, 2-ethylhexyl acrylate,n-hexyl acrylate, n-octyl acrylate, n-decyl acrylate, n-dodecyl acrylateor 3,5,5-trimethylhexyl acrylate.

In the case where a mixture of alkyl acrylates is used to produce thecopolymers (I) and/or (II), use will be made of an amount by weight ofn-alkyl acrylate at least equal to 10% by weight of the mixture of alkylacrylates and preferably an amount of between 20% and 80%.

As above, use may be made, to form the copolymers (I) and/or (II), of amixture of identical or different alkyl acrylates.

According to the present invention, it is preferable to use n-alkylacrylates and very particularly n-octyl acrylate to form the copolymers(I) and (II).

If a mixture of alkyl acrylates is used to form the copolymers (I)and/or (II), use will preferably be made of 20% to 80% by weight ofn-octyl acrylate and preferably of 80% to 20% by weight of n-butylacrylate.

Mention will be made, as illustration of alkyl methacrylates which canbe used to form the shell grafted onto the crosslinked elastomeric coreaccording to the present invention, of ethyl methacrylate, n-propylmethacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutylmethacrylate and very particularly methyl methacrylate.

According to the present invention, the crosslinking agent used to formthe copolymer (I) can in particular be chosen from derivativespossessing at least two double bonds of the vinyl type or alternativelypossessing one or a number of double bonds of the vinyl type and atleast one double bond of the allyl type. Use will preferably be made ofcompounds possessing, in their molecules, a majority of double bonds ofthe vinyl type.

Mention will be made, as illustration of such crosslinking agents, ofdivinylbenzenes, polyalcohol (meth)acrylates, such astrimethylolpropane, triacrylate or trimethacrylate, allyl acrylate ormethacrylate, alkylene glycol diacrylates or dimethacrylates having 2 to10 carbon atoms in the alkylene chain and in particular ethylene glycoldiacrylate or dimethacrylate, 1,4-butanediol diacrylate ordimethacrylate or 1,6-hexanediol diacrylate or dimethacrylate orpolyoxyalkylene glycol diacrylate or dimethacrylate of formula

in which X represents a hydrogen atom or the methyl radical, n is aninteger ranging from 2 to 4 and p is an integer ranging from 2 to 20 andin particular polyoxyethylene glycol diacrylate or dimethacrylate inwhich the polyoxyethylene radical has a molecular mass of approximately400 (abovementioned formula with n=2 and p=9).

According to the present invention, the grafting agent used to form thecopolymer (II) can be in particular chosen from derivatives possessingat least two double bonds of the allyl type or alternatively possessingone or a number of double bonds of the allyl type and at least onedouble bond of the vinyl type.

Use will preferably be made of compounds possessing, in their molecules,a majority of double bonds of the allyl type.

Mention will be made, as illustration, of such grafting agents, ofdiallyl maleate, diallyl itaconate, allyl methacrylate or acrylate,triallyl cyanurate, triallyl isocyanurate, diallyl terephthalate ortriallyl trimesate.

According to an alternative form in accordance with the invention, thenucleus of the crosslinked elastomeric core can be composed entirely ofa polyorganosiloxane rubber obtained by emulsion polymerization of anorganosiloxane in the presence of a crosslinking agent and, optionally,of a grafting agent.

Mention may be made, as illustration of organosiloxanes, of cyclicsiloxanes composed of rings having a number of Si—C ring members rangingfrom 3 to 6, such as hexamethylcyclotrisiloxane,octamethylcyclotetrasiloxane, dodecamethylcyclotetrasiloxane oroctaphenylcyclotetrasiloxane.

Mention may be made, as crosslinking agent which can be sued, of acrosslinking agent of the tri- or tetrafunctional silane type, such as,for example, trimethoxysilane or tetraethoxysilane.

Use will preferably be made, as grating agent, of amethacryloyloxysiloxane of formula:

in which R₁ represents a methyl, ethyl, propyl or phenyl group, R²represents a hydrogen atom or a methyl group, n has a value 0, 1 or 2and p is a number ranging from 1 to 6.

Mention may be made, as illustration of methacryloyloxysiloxane, of:

β-methacryloyloxyethyldimethoxymethylsilane,

γ-methacryloyloxypropylmethoxydimethylsilane,

γ-methacryloyloxypropyldimethoxysilane,

γ-methacryloyloxypropyltrimethoxysilane,

γ-methacryloyloxypropylethoxydiethylsilane,

γ-methacryloyloxypropyldiethoxymethylsilane, and

δ-methacryloyloxybutyldiethoxymethylsilane.

The polyorganosiloxane rubber can be produced by a process described,for example, in European Patent EP 0,326,038. Use will very particularlybe made of the procedure described in the example of reference 1 of thesaid patent, which makes it possible to obtain apolyoctamethylcyclotetrasiloxane rubber latex.

According to this alternative form, the crosslinked elastomeric core cancontain no more than 40% by weight of a nucleus composed of apolyorganosiloxane rubber as described above.

The invention also relates to a composition comprising a thermoplasticpolymer and the impact additive as defined above.

The thermoplastic polymer can be composed of one or a number of polymersof the polycondensates type, in particular polyamides,polyetheresteramides (PEBAX), polyesters, such as polybutyleneterephthalate, polycarbonates or alloys of the abovementioned polymers,such as alloys of polycarbonates and of the polyesters, such as XENOY.The thermoplastic polymer can also be composed of one or a number ofpolymers chosen from the group formed by poly(alkyl methacrylate)s andin particular poly(methyl methacrylate) or by a vinyl chloridehomopolymers, which can optionally be superchlorinated, and copolymerswhich result from the copolymerization of vinyl chloride with one or anumber of ethylenically unsaturated comonomers and which contain atleast 80% by weight of polymerized vinyl chloride. Examples of monomerswhich are suitable for the preparation of such copolymers are inparticular vinylidene halides, such as vinylidene chloride or fluoride,vinyl carboxylates, such as vinyl acetate, vinyl propionate or vinylbutyrate, acrylic and methacrylic acids and the nitriles, amides andalkyl esters which derive therefrom, in particular acrylonitrile,acrylamide, methacrylamide; methyl methacrylate, methyl acrylate, butylacrylate, ethyl acrylate or 2-ethylhexyl acrylate, vinylaromaticderivatives, such as styrene or vinylnaphthalene, or olefins, such asbicyclo[2.2.1]hept-2-ene, bicyclo[2.2.1]hepta-2,5-diene, ethylene,propene or 1-butene.

The thermoplastic polymer can also be composed of a homopolymer of avinylidene halide, such as 1,1-dichloroethylene or 1,1-difluoroethylene.

The thermoplastic polymer is preferably a vinyl chloride homopolymer ora poly(butylene terephthalate).

The preferred content of impact additive incorporated in thethermoplastic polymer is between 1 and 30 parts by weight, andpreferable between 5 and 10 parts by weight, per 100 parts by weight ofthe thermoplastic polymer used.

In order to describe the molecular mass of the impact additive, it ispossible to define a viscosity in the molten state which varies in thesame sense. The said viscosity in the molten state may be situated in afairly wide range, provided that the impact additive is well dispersedduring the operations in which the resin composition, including the saidadditive, is made use of. As representative magnitude of this viscosityin the molten state, the value of the resisting torque of a Brabenderrheometer containing 50 g of impact additive and operating at atemperature of 200° C. with a rotational speed of its rotors equal to 40revolutions per minute may suitably be taken, the torque beingdetermined after holding at 200° C. for 20 minutes. Appropriate valuesof the viscosity in the molten state for the impact additive correspondto values of the abovementioned torque of between 600 and 4000 m.g. Inthe case of resin compositions for which the thermoplastic polymer is apolymer containing at least 80% by weight of polymerized vinyl chloride,preferred viscosity values in the molten state for the impact additivecorrespond to values of the said torque ranging from 800 to 3000 m.g.and very particularly from 1000 to 2500 m.g.

Another subject of the invention is a process for producing the saidimpact additive.

One process comprises the preparation, in a first state, of acrosslinked core composed of a nucleus and of a covering and then, in asecond stage, a poly(alkyl methacrylate) shell is grafted onto the saidcrosslinked core obtained in the first stage.

According to a preferred method, the crosslinked core, composed of anucleus and of a covering, is prepared and the grafting operation iscarried out by using emulsion polymerization techniques. In this case,the following procedure can be used.

In a first stage, an emulsion is prepared which contains, per part byweight of monomers to be polymerized, 1 to 10 parts of water, 0.001 to0.03 parts of an emulsifying agent, a major portion of the n-alkylacrylate or of the mixture of alkyl acrylates as defined above to bepolymerized in order to form the said core and at least onepolyfunctional crosslinking agent. The reaction mixture thus formed isstirred and maintained at a temperature ranging from 55° C. to 65° C.and preferably at a temperature in the region of 60° C. 0.001 to 0.5parts of a catalyst which generates free radicals is then added and thereaction mixture thus formed is maintained at a temperature of, forexample, between ambient temperature and 100° C. and with stirring for aperiod sufficient to obtain a virtually complete conversion of themonomers. The minor portion of n-alkyl acrylate or of the mixture ofalkyl acrylates and the grafting agent, as well as, at the same time,0.001 to 0.005 part of a catalyst which generates free radicals, arethen added simultaneously to the phase thus obtained.

This second operation of the first stage, which comprises the productionof the covering, is generally carried out at a temperature greater thanthat used for the preparation of the nucleus. This temperature is notgreater than 100° C. and preferably between 60° C. and 90° C.

An alternative form of this first stage comprises the production of thecrosslinked core in a single operation by simultaneously introducing thecrosslinking agent and the grafting agent (or a compound which playsboth the crosslinking role and the grafting role) into the reactionmixture.

In a second stage, the said core is grafted with an alkyl methacrylate.To do this, an appropriate amount of the said methacrylate is added tothe reaction mixture resulting from the first stage, in order to obtaina grafted copolymer containing the desired content of grafted chains, aswell as, if appropriate, additional amounts of emulsifying agent and ofa radical catalyst also within the ranges defined above, and the mixturethus formed is maintained at a temperature within the abovementionedrange, with stirring, until virtually complete conversion of thegrafting monomers is obtained.

Use may be made, as emulsifying agent, of any one of the knownsurface-active agents, whether anionic, nonionic or even cationic. Inparticular, the emulsifying agent may be chosen from anionic emulsifyingagents, such as sodium or potassium salts of fatty acids, in particularsodium laurate, sodium stearate, sodium palmitate, sodium oleate, mixedsulphates of sodium or of potassium and of fatty alcohols, in particularsodium lauryl sulphate, sodium or potassium salts of sulphosuccinicesters, sodium or potassium salts of alkylarylsulphonic acids, inparticular sodium dodecylbenzenesulphonate, and sodium or potassiumsalts of fatty monoglyceride monosulphonates, or alternatively fromnonionic surfactants, such as the reaction products of ethylene oxideand of alkylphenol or of aliphatic alcohols, alkylphenols. Use may alsobe made of mixtures of such surface-active agents, if need be.

The catalysts capable of being employed, both in the abovementionedfirst emulsion polymerization stage and in the abovementioned secondemulsion polymerization stage, are compounds which give rise to freeradicals under the temperature conditions chosen for the polymerization.These compounds can in particular be peroxide compounds, such ashydrogen peroxide; alkali metal persulphates and in particular sodium orpotassium persulphate; ammonium persulphates; percarbonates;peracetates, perborates; peroxides such as benzoyl peroxide or lauroylperoxide; or hydroperoxides such as cumene hydroperoxide,diisopropylbenzene hydroperoxide, para-menthane hydroperoxide ortert-butyl hydroperoxide.

However, it is preferable to use catalytic systems of redox type formedby the combination of a peroxide compound, for example as mentionedabove, with a reducing agent, in particular such as alkali metalsulphite, alkali metal bisulphite, sodium formaldehyde sulphoxylate(NaHSO₂, HCHO), ascorbic acid, glucose, and in particular those of thesaid catalytic systems which are water-soluble, for example potassiumpersulphate/sodium metabisulphite or alternatively diisopropylbenzenehydroperoxide/sodium formaldehyde sulphoxylate.

It is also possible to add, to the polymerization mixture of one and/orother of the stages, chain-limiting compounds, and in particularmercaptans such as tert-dodecyl mercaptan, isobutyl mercaptan, n-octylmercaptan, n-dodecyl mercaptan or isooctyl mercaptopropionate, for thepurpose of controlling the molecular mass of the core and/or of thechains grafted onto the nucleus, or alternatively compounds such asphosphates, for the purpose of controlling the ionic strength of thepolymerization mixture.

The reaction mixture obtained on conclusion of the second emulsionpolymerization stage, which is composed of an aqueous emulsion of theadditive according to the invention, is then treated in order toseparate the said additive therefrom. To do this, it is possible, forexample, to subject the emulsion, according to the surfactant used, to acoagulating treatment by bringing into contact with a saline solution(CaCl₂ or AlCl₃) or a solution acidified with concentrated sulphuricacid and then to separate, by filtration, the solid product resultingfrom the coagulating, the said solid product then being washed and driedto give a graft copolymer as a powder. It is also possible to recoverthe additive contained in the emulsion by using a spray-dryingtechnique.

The resulting additive exists in the form of a powder, the particle sizeof which can range from a few microns, for example 0.05 to 5 microns, to200 to 300 microns, the said particle size depending on the techniqueused to separate the graft copolymer from the emulsion polymerizationmixture.

The composition according to the invention can be prepared by any methodwhich makes it possible to produce a homogeneous mixture containing athermoplastic polymer, the impact additive according to the inventionand optionally other additives. It is possible, for example, to dry-mixthe ingredients constituting the resin composition, then to extrude theresulting mixture and to reduce the extrudate to pellets. When thethermoplastic polymer is obtained by emulsion polymerization, it may beconvenient to mix the emulsion containing the additive according to theinvention with the emulsion of the thermoplastic polymer and to treatthe resulting emulsion in order to separate therefrom the solid productwhich it contains, as described above with respect to the separation ofthe additive.

The additives other than the impact additive, which may optionally bepresent in the resin compositions according to the invention are inparticular those such as pigments, dyes, plasticizers, antioxidants,heat stabilizers, processing additives or lubricants.

The PVC composition obtained according to the present invention exhibitsexcellent impact strength at ambient temperature as well as attemperatures as low as −30° C. or even −40° C.

The composition of the present invention can advantageously be used toproduce sections or claddings used in particular in the buildingindustry or alternatively to produce pipes which can be used forconveying water.

The following examples illustrate the invention.

EXAMPLE 1 (according to the invention)

The preparation is carried out in a 2-liter reactor equipped with astirrer device and a temperature recorder and provided with a jacketthrough which passes a heat-transfer fluid for maintaining thetemperature of the reactor.

1) Preparation of the crosslinked elastomeric core of the impactadditive:

800 g of demineralized water and 2.46 g of disodium phosphate areintroduced, after having degassed with nitrogen, into the reactordescribed above, maintained at ambient temperature and with stirring,and then 20.58 g of sodium lauryl sulphate are dissolved in this mixtureas emulsifying agent.

The temperature of the contents of the reactor is then brought to 57° C.and 423 g of n-octyl acrylate and 5.08 g of 1,4-butanediol diacrylateare then added simultaneously to the said contents, while maintainingthis temperature.

The temperature of the reactor is brought to 63° C. and 0.41 g of sodiummetabisulphite in 5.59 ml of water and 0.62 g of potassium persulphatein 6.58 ml of water are ten added to the reaction mixture as catalyticsystem. The reaction is then allowed to continue for 2 hours, thetemperature of the reactor is then brought to 80° C. and 47 g of n-octylacrylate, 2.36 g of diallyl maleate and 0.8 g of potassium persulphateare then added simultaneously.

The temperature of the reactor is maintained at 80° C. for 1 hour. Acrosslinked elastomeric core is obtained, with a conversion of 98%,consisting of:

1) 89.66% by weight of a nucleus composed of an n-octyl acrylate/1,4,-butanediol diacrylate copolymer (I) and of

2) 10.34% by weight of a covering composed of an n-octylacrylate/diallyl maleate copolymer (II).

this core contains, in moles, 1% of 1,4-butanediol diacrylate and 0.47%of diallyl maleate.

2) Grafting of the methyl methacrylate onto the crosslinked elastomericcore:

118 g of methyl methacrylate are continuously added over one hour, withstirring, to the reaction mixture obtained above maintained at 80° C.1.7 g of diisopropylbenzene hydroperoxide in 78 ml of water and 0.2 g ofsodium formaldehyde sulphoxylate in 4 ml of water are also added at thesame time.

The contents of the reactor are maintained at 80° C. for 1.5 hours,after the beginning of the introduction of the methyl methacrylate, and0.31 g of tert-butyl hydroperoxide and 0.8 g of sodium metabisulphite in15 ml of water are added to the said contents. The reaction mixture isthen maintained at 80° C. for one hour. At the end of this period, thecontents of the reactor are cooled to ambient temperature and thegrafted copolymer latex produced, the mean particle diameter of which is0.08 μm, is coagulated in a saline solution acidified with concentratedsulphuric acid. The coagulated product is then filtered, washed and thendried to give a powder constituting the impact additive.

The conversion of the methyl methacrylate during the grafting is 99%.The impact additive contains a proportion of grafted poly(methylmethacrylate) chains representing 19.82% by weight of the additive andhas a viscosity in the molten state corresponding to a value equal to1400 m.g. of the torque of the Brabender rheometer operating under theconditions defined above.

EXAMPLE 2 (in accordance with the invention)

1) Preparation of the crosslinked elastomeric core of the impactadditive:

2000 g of demineralized water and 5.85 g of disodium phosphate areintroduced, after having degassed with nitrogen, into a 5-liter reactorequipped as described in Example 1 maintained at room temperature andwith stirring and then 245 g of sodium lauryl sulphate are dissolved inthis mixture as emulsifying agent.

The temperature of the contents of the reactor are then brought to 57°C. and 904.5 g of n-octyl acrylate, 301.5 g of n-butyl acrylate and 14.4g of 1,4-butanediol diacrylate are then simultaneously added to the saidcontents, while maintaining this temperature.

The temperature of the reactor is brought to 63° C. and 22.6 g of sodiumbisulphite in 10 ml of water and 1.60 g of potassium persulphate in33.83 ml of water are then added to the reaction mixture as catalyticsystem. The reaction is then allowed to continue under adiabaticconditions for 2 hours, the temperature of the reactor is then broughtto 80° C. and 100.5 g of n-octyl acrylate, 33.5 g of n-butyl acrylate,6.71 g of diallyl maleate and 0.17 g of potassium persulphate in 3.73 mlof water are then added simultaneously.

The temperature of the reactor is maintained at 80° C. for 1 hour.

A crosslinked elastomeric core is obtained, with a conversion of 95.72%,consisting of:

1) 89.66% by a nucleus composed of an n-octyl acrylate/n-butylacrylate/1,4-butanediol diacrylate copolymer (I) and of

2) 10.34% by weight of a covering composed of an n-octylacrylate/n-butyl acrylate/dialkyl maleate copolymer (II).

2) Grafting of the methyl methacrylate onto the crosslinked elastomericcore:

335.3 g of the methyl methacrylate are continuously added over one hour,with stirring, to the reaction mixture obtained above; maintained at 80°C. 4.3 g of diisopropylbenzene hydroperoxide in 195.7 ml of water and0.40 g of sodium formaldehyde sulphoxylate in 9.28 ml of water are alsoadded at the same time.

The contents of the reactor are maintained at 80° C. for 1.5 hours,after the beginning of the introduction of the methyl methacrylate, and0.24 g of tert-butyl hydroperoxide and 4.6 g of sodium bisulphite in 17ml of water are added to the said contents. The reaction mixture is thenmaintained at 80° for one hour. At the end of this period, the contentsof the reactor are cooled to ambient temperature and the graftedcopolymer latex produced is coagulated in a saline solution acidifiedwith concentrated sulphuric acid. The coagulated product is thenfiltered, washed and then dried to give a powder constituting the impactadditive.

The conversion of the methyl methacrylate during the grafting is 98.27%.The impact additive contains a proportion of grafted poly(methylmethacrylate) chains representing 19.77% by weight of the additive andhas a viscosity in the molten state corresponding to a value equal to1500 m.g. of the torque of the Brabender rheometer operating under theconditions defined above.

In Examples 3 to 7, the preparation is carried out according to the sameoperating conditions of Example 1, using, for the crosslinkedelastomeric core as for the shell grafted onto the said core, the samereactants (grafting and crosslinking agents, catalysts, emulsifiers, andthe like) in identical amounts by weight, except that, for thepreparation of the crosslinked elastomeric core, use is made ofidentical amounts by weight of an alkyl acrylate other than n-octylacrylate, to produce the copolymers (I) and (II).

In Example 3, n-heptyl acrylate is used.

In Example 4, n-hexyl acrylate is used.

In Example 5, n-pentyl acrylate is used.

In Example 6 (not in accordance with the invention), n-butyl acrylate isused.

In Example 7 (not in accordance with the invention), 2-ethylhexylacrylate is used.

Impact additives are obtained which contain, as in Example 1, acrosslinked elastomeric core, obtained with yields greater than 98%,consisting of:

1) approximately 90% by weight of a nucleus composed of an alkylacrylate/1,4-butanediol diacrylate copolymer (I) and of

2) approximately 10% by weight of a covering composed of an alkylacrylate copolymer (II) identical to that used in 1/dialkyl maleate, anda shell, made of poly(methyl methacrylate), grafted onto the said coreand representing approximately 20% by weight of the impact additive.

EXAMPLE 8 (in accordance with the invention)

1) Preparation of the crosslinked elastomeric core of the impactadditive:

750 g of demineralized water, 2.46 g of disodium phosphate and 0.41 g ofsodium metabisulphite in 7.59 ml of water are introduced, after havingdegassed with nitrogen, into the reactor described in Example 1,maintained at ambient temperature and with stirring, and then 100 g ofsodium lauryl sulphate are dissolved in this mixture as emulsifyingagent.

The temperature of the contents of the reactor is then brought to 57° C.and 117.5 g of a n-octyl acrylate and 1.21 g of allyl methacrylate arethen added simultaneously to the said contents, while maintaining thistemperature.

The temperature of the reactor is brought to 63° C. and 0.13 g ofpotassium persulphate in 2.67 ml of water is then added to the reactionmixture. The reaction is then allowed to continue for 45 minutes, thetemperature of the reactor is then brought to 80° C. and 353 g ofn-butyl acrylate, 5.21 g of allyl methacrylate and 0.5 g of potassiumpersulphate in 10.7 ml of water are then added simultaneously.

The temperature of the reactor is maintained at 80° C. for 1 hour.

A crosslinked elastomeric core is obtained, with a conversion of 99.9%,consisting of

1) 24.89% by weight of a nucleus composed of an n-octyl acrylate/allylmethacrylate copolymer (I) and of

2) 75.11% by weight of a covering composed of an n-butyl acrylate/allylmethacrylate copolymer (II).

2) Grafting of the methyl methacrylate onto the crosslinked elastomericcore:

118 g of methyl methacrylate are continuously added over one hour, withstirring, to the reaction mixture obtained above maintained at 80° C.1.7 g of diisopropylbenzene hydroperoxide in 78 ml of water and 0.42 gof sodium formaldehyde sulphoxylate in 9.58 ml of water are also addedat the same time.

The contents of the reactor are maintained at 80° C. for 1.5 hours,after the beginning of the introduction of the methyl methacrylate, and0.33 g of tert-butyl hydroperoxide and 0.08 of sodium metabisulphite in10 ml of water are added to the said contents. The reaction mixture isthen maintained at 80° for one hour. At the end of this period, thecontents of the reactor are cooled to ambient temperature and thegrafted copolymer latex produced is coagulated in a saline solutionacidified with concentrated sulphuric acid. The coagulated product isthen filtered, washed and then dried to give a powder constituting theimpact additive.

The conversion of the methyl methacrylate during the grafting isquantitative. The impact additive contains a proportion of poly(methylmethacrylate) graft chains representing 19.83% by weight of the additiveand has a viscosity in the molten state corresponding to a value equalto 1760 m.g. of the torque of the Brabender rheometer operating underthe conditions defined above.

EXAMPLE 9 (in accordance with the invention)

The preparation is carried out in a 2-liter reactor equipped with astirring device and a temperature recorder and provided with a jacketthrough which passes a heat-transfer fluid for maintaining thetemperature of the reactor.

1) Preparation of the crosslinked elastomeric core of the impactadditive:

787.5 g of demineralized water, 2.58 of disodium phosphate and 0.42 g ofsodium metabisulphite in 7.98 ml of water are introduced, after havingdegassed with nitrogen, into the reactor described above maintained atambient temperature and with stirring and then 105 g of sodium laurylsulphate are dissolved in this mixture as emulsifying agent.

The temperature of the contents of the reactor are then brought to 57°C. and 493.50 g of n-octyl acrylate and 3.38 g of allyl methacrylate arethen added simultaneously to the said contents, while maintaining thistemperature.

The temperature of the reactor is brought to 63° C. and 0.66 g ofpotassium persulphate in 14 ml of water is then added to the reactionmixture. The reaction is then allowed to continue for 2 hours and thenthe temperature of the reactor is brought to 80° C.

The temperature of the reactor is maintained at 80° C. for 1 hour. Acrosslinked elastomeric core is obtained, with a conversion of 98.95%,consisting of an n-octyl acrylate/allyl methacrylate copolymer (I).

This core contains, in moles, 1% of allyl methacrylate.

2) Grafting of methyl methacrylate onto the crosslinked elastomericcore:

118 g of methyl methacrylate are continuously added over one hour, withstirring, to the reaction mixture obtained above maintained at 80° C.1.7 g of diisopropylbenzene hydroperoxide in 78 ml of water and 0.42 gof sodium formaldehyde sulphoxylate in 9.58 ml of water are also addedat the same time.

The contents of the reactor are maintained at 80° C. for 1.5 hours,after the beginning of the introduction of the methyl methacrylate, and0.33 g of tert-butyl hydroperoxide and 0.08 g of sodium metabisulphitein 10 ml of water are added to the said contents. The reaction mixtureis then maintained at 80° C. for one hour. AT the end of this period,the contents of the reactor are cooled to ambient temperature and thegrafted copolymer latex produced is coagulated in a saline solutionacidified with concentrated sulphuric acid. The coagulated product isthen filtered, washed and then dried to give a powder constituting theimpact additive.

The conversion of the methyl methacrylate during the grafting is 96.17%.The impact additive contains a proportion of poly(methyl methacrylate)graft chains representing 19.82% by weight of the additive and has aviscosity in the molten state corresponding to a value equal to 1300g.m. of the torque of the Brabender rheometer operating under theconditions defined above.

EXAMPLE 10

1. Preparation of the crosslinked elastomeric core of the impactadditive:

First stage: Preparation of the seed.

The preparation is carried out in a 5-liter reactor equipped with astirrer, a temperature recorder and a jacket through which passes aheat-transfer fluid for maintaining the temperature of the reactor.

1,100 g of demineralized water and 0.95 g of sodium hydrogencarbonate in95 g of water are introduced, after having degassed with nitrogen, intothis reactor maintained at ambient temperature and with stirring andthen 4.76 g of sodium diocytl sulphosuccinate are dissolved in thismixture as emulsifying agent.

The temperature of the contents of the reactor is then brought to 57° C.and, while maintaining this temperature, 119 g of n-octyl acrylate and2.56 g of 1,4-butanediol diacrylate are then added simultaneously to thesaid contents.

The temperature of the reactor is brought to 70° C. and 2.62 g ofpotassium persulphate, dissolved in 65 g of water, are added to thereaction mixture.

After an induction time of approximately 10 minutes, the temperaturerises to 76° C. An emulsified mixture composed of 663 g of demineralizedwater, 0.66 g of sodium hydrogencarbonate in 66 g of water, 6.43 g ofsodium dioctyl sulphosuccinate as emulsifying agent, 1,071 g of n-octylacrylate and 23.05 g of 1,4-butanediol diacrylate is then added to thereactor over a period of two hours. The temperature is maintained at 70°C. during the addition period. The temperature is then increased to 90°C. and maintained for one hour.

A crosslinked elastomeric seed, denoted by “emulsion (A)”, is obtained,with a conversion of 99%, consisting of latex particles with a diameterof 0.130 μm.

Second stage: Preparation of the core.

The preparation is carried out in a 5-liter reactor equipped with astirrer, a temperature recorder and a jacket through which passes aheat-transfer fluid for maintaining the temperature of the reactor.

An emulsified premix (B) is prepared composed of 660 g of demineralizedwater, 0.66 g of sodium hydrogencarbonate in 66 g of water, 6.43 g ofsodium dioctyl sulphosuccinate as emulsifying agent, 1,071 g of n-octylacrylate and 23.05 g of 1,4-butanediol diacrylate.

The reactor being maintained at room temperature and with stirring,1,000 g of demineralized water and 1 g of sodium hydrogencarbonate in100 g of water are introduced, after having degassed with nitrogen, andthen 338.88 g of the emulsion (A) obtained during the first stage aredissolved in this mixture.

The temperature of the contents of the reactor is then brought to 57° C.and 120 g of premix (B) are then added, while maintaining thistemperature.

The temperature of the reactor is brought to 70° C. and 2.14 g ofpotassium persulphate, dissolved in 65 g of water, are added to thereaction mixture.

After an induction time of approximately 10 minutes, the temperaturerises to 76° C. 1,505 g of the premix (B) are then added to the reactorover a period of 110 minutes. 5.95 g of diallyl maleate are then addedto the remaining 200 g of premix B and the combined mixture is added tothe contents of the reactor, while still maintaining the temperature at70° C., over a period of 10 minutes.

The temperature is then increased to 90° C. and maintained for one hour.

The elastomeric core is obtained, with conversion of 99%, consisting oflatex particles with a Coulter diameter of 0.270 μm.

2. Grafting of methyl methacrylate on to the crosslinked elastomericcore

0.54 g of potassium persulphate, dissolved in 30 g of water, is added,with stirring, to the reaction mixture obtained above maintained at 70°C. An emulsified mixture (C), composed of 200 g of demineralized water,0.35 g of sodium hydrogencarbonate in 35 g of water, 1.34 g of sodiumdioctyl sulphosuccinate as emulsifying agent, 267 g of methylmethacrylate and 29.75 g of ethyl acrylate, is then added continuouslyover 45 minutes.

On completion of this addition, the contents of the reactor aremaintained at 90° C. for one hour. AT the end of this period, thecontents of the reactor are cooled to ambient temperature.

A grafted copolymer latex is obtained, with a conversion of 98.3%, themean particle diameter of which is 0.285 μm.

this latex is then coagulated in a calcium chloride solution. Thecoagulated product is then filtered, washed and then dried to give apowder which constitutes the impact additive.

This additive has a viscosity in the molten state corresponding to avalue equal to 890 m.g. of the Brabender rheometer torque operatingunder the conditions defined above.

EXAMPLE 11 (not in accordance with the invention)

The preparation is carried out according to the same operatingconditions as EXAMPLE 10, use being made, for the preparation of theseed, of the crosslinked elastomeric core and for the shell grafted ontothe said core, of the same reactants (grafting and crosslinking agents,catalysts, emulsifiers, and the like) in identical amounts by weight,except that, for the preparation of the seed and of the crosslinkedelastomeric core, use is made of an amount by weight identical of2-ethylhexyl acrylate in place of n-octylacrylate.

A grafted copolymer latex is obtained, with a conversion of 99.1%, themean particle diameter of which is 0.315 μm.

This latex is then coagulated in a calcium chloride solution. Thecoagulated product is then filtered, washed and then dried to give apowder which constitutes the impact additive.

This additive has a viscosity in the molten state corresponding to avalue equal to 1,725 m.g. of the Brabender rheometer torque operatingunder the conditions defined above.

EXAMPLE 12 (not in accordance with the invention

The preparation is carried out according to the same operatingconditions as Example 10, use being made, for the preparation of theseed, of the crosslinked elastomeric core and for the shell grafted ontothe said core, of the same reactants (grafting and crosslinking agents,catalysts, emulsifiers, and the like) in identical amounts by weight,except that, for the preparation of the seed and of the crosslinkedelastomeric core, use is made of an identical amount by weight of butylacrylate in place of n-octyl acrylate and of an identical molar amount,with respect to the butyl acrylate monomer, of 1,4-butanedioldiacrylate.

A grafted copolymer latex is obtained, with a conversion of 98.6%, themean particle diameter of which is 0.335 μm.

This latex is then coagulated in a calcium chloride solution. Thecoagulated product is then filtered, washed and then dried to give apowder which constitutes the impact additive.

This additive has a viscosity in the molten state corresponding to avalue equal to 1,725 m.g. of the Brabender rheometer torque operatingunder the conditions defined above.

Preparation and characteristics of the resin compositions according tothe invention:

1. The description is given below of the preparation of a PVC-basedresin composition and the impact strength characteristics of testspecimens manufactures from this resin composition are given.

The preparation is carried out at 25° C., in a mixer of the Papenmeirtype, of a composition containing (parts by weight):

100 parts of a vinyl chloride homopolymer of E-value=67,

2.5 parts of lead phosphite,

1.5 parts of calcium stearate,

6 parts of calcium carbonate,

4 parts of TiO₂,

1 part of a processing aid (Metablen P550, sold by the company MetablenB.V.),

0.2 part of 12 stearic acid,

0.3 part of Loxiol G60 (internal lubricant),

4 parts of polyethylene waxes (external lubricant) and,

x parts of an impact additive prepared according to one of Examples 1 to9.

From the composition thus obtained, test specimens are prepared forcarrying out the impact strength determination tests.

To prepare the test specimens for the Charpy impact tests, the PVC resincompositions resulting from the mixtures of the abovementionedingredients are calendered at 175° C. for 6 minutes on a calender of theSchwanbenthan type and then moulded at 190° C. on a Derragon press, for5 minutes under a pressure of 200 bar, in the form of panels, the saidpanels being cooled in the press.

The test specimens are cut out using a circular saw and then a notcherfor the notched Charpy impact tests, according to BS standard 2872.

The thickness of the test specimens, the shape of which is thatstipulated by the abovementioned standard, is 2.5 mm.

To prepare the test specimens for the low-temperature impact strengthtest according to ISO standard 6603.2 1989 (F), the resin compositiondefined above is mixed in a twin-screw extruder of the Krauss-Maffei KMD25 type, then introduced into a die, which makes it possible to obtain astrip with a thickness of 1 mm, and then cut up into 7 cm×7 cm squares.

The results are combined in the tables below.

In Table 1, the source of the impact additive (example) and its contentin the PVC resin composition as described above, in parts by weight per100 parts by weight of the said resin (phr), have been shown in the“impact additive” column. The “Charpy impact” strength tests are carriedout according to BS standard 2782 at a temperature of 23°±1° C. Thefracture energy is calculated by taking the mean of the ductile andbrittle fracture energies.

The low-temperature impact strength tests are reported in Table 2. As inTable 1, the source of the said impact additive and its content (phr) inthe PVC resin composition have been shown in the “impact additive”column.

In Tables 1 and 2, the compositions 9(c), 10(c), 11(c), 12(c), 21(c),and 22(c) are not in accordance with the invention.

2. The preparation of a resin composition based on poly(butyleneterephthalate)(PBT) is described below and the impact strengthcharacteristics of test specimens manufactured from this resincomposition are given.

The preparation is carried out at 25° C. of a resin compositionaccording to the invention containing (parts by weight):

80 parts of a poly(butylene terephthalate) homopolymer (Calanex 1700A,sold by the company Hoechst Calenese),

20 parts of an impact additive prepared according to one of Examples 10to 12.

The mixture is dried for at least 10 hours under a vacuum of 1 bar at80° C.

This mixture is homogenized by extrusion on a Buss PR46 Ko-Knewader,followed by granulation of the rod obtained. The extrusion conditionsare as follows:

Ko-Kneader temperature of the screw: 230° C. temperature zone 1: 260° C.temperature zone 2: 250° C. speed: 120 revolutions/ minute Extrudertemperature of the screw: 240° C. temperature zone 1: 240° C. die: 230°C. speed: 94 revolutions/ minute

The test specimens for the Izod impact tests are prepared by injectionmoulding the granules obtained above on a Visumat 5000 injection press.These granules are dried for at least 10 hours under a vacuum of 1 barat 80° C. Injection is carried out under the following conditions:

Injection temperature: 240° C. Injection rate: 10% Injection pressure:80 bar Hold pressure: 50 bar Hold time: 20 seconds

These test specimens, the shape and thickness of which are thosedescribed in ISO standard 180, are then notched.

The results obtained are presented in the table below. In Table 3, thesource of the impact additive (Example) has been shown in the “impactadditive” column. The fracture energy according to ISO standard 180 at atemperature of 25°±1° C., a calculated by taking the mean of the ductileand brittle fractures with respect to ten test specimens, has beenreported in the “ambient temperature impact” column. The fracture energyaccording to ISO standard 180 at a temperature of −20° C. ±1° C.,calculated by taking the mean of the ductile and brittle fractures withrespect to ten test specimens, has been reported in the “cold impact”column.

3. The preparation of a resin composition based onpoly(1,1-difluoroethylylene)(PVDF) is described below and the impactstrength characteristics of test specimens manufactured from this resincomposition are given.

The preparation is carried out at 25° C. of a resin compositionaccording to the invention containing (parts by weight):

95 parts of a poly(1,1-difluoroethylene) homopolymer (Kynar 1000, soldby the company ELF ATOCHEM S.A.),

5 parts of an impact additive prepared according to Examples 10 and 12.

The mixture is homogenized by extrusion on a Werner 40, followed bygranulation of the rod obtained. The extrusion conditions are asfollows:

temperature zone 1: 195° C. temperature zone 2: 230° C. temperature zone3: 215° C. temperature zone 4: 240° C.

The test specimens for the Izod impact and Charpy impact tests areprepared by injection moulding the granules obtained above on a Visumat5000 injection press in the shape of sheet of 100 mm×100 mm. These testspecimens, the shape and thickness of which are those described in ISOstandard 179 and ISO standard, are punched with tool.

The results obtained are presented in the table 4. In this Table 4, thesource of the impact additive (Example) has been shown in the “impactadditive” column. The fracture energy according to ISO standard 180 at atemperature of 25° C.±1° C., calculated by taking the mean of theductile and brittle fractures with respect to ten test specimens, hasbeen reported in the “ambiant temperature impact IZOD” column. Thefracture energy according to ISO standard 179 at a temperature of 23°C.±1° C., calculated by taking the mean of the ductile and brittlefractures with respect to ten test specimens, has been reported in the“ambient temperature impact CHARPY”.

The fracture energy according to ISO standard 179 at a temperature of−40° C.±1° C., calculated by taking the mean of the ductile and brittlefractures with respect to ten test specimens, has been reported in the“cold impact, CHARPY” column.

TABLE 1 CHARPY IMPACT IMPACT ADDITIVE Fracture % of COM- Source Contentenergy DUCTILE POSITION (Example) (phr) (KJ/M²) Fracture  1 1 7 34.2 60 2 1 7.5 >52 100  3 3 7 48 100  4 3 7.5 >52 100  5 4 7 46.5 100  6 47.5 >52 100  7 2 7 43.1 80  8 2 8 52 100  9(c) 6 7 15.7 0 10(c) 6 8 48.690 11(c) 7 7 12.4 0 12(c) 7 7.5 39 70 13 8 7 15.2 0 14 8 7.5 >51 100 159 7 >43 100 16 9 7.5 >52 100

TABLE 2 LOW-TEMPERATURE IMPACT IMPACT ADDITIVE Temperature Fracture COM-Source Content of the test Energy POSITION (example) (phr) (in ° C.) (inKJ/M²) 17 1 6 −10 23.1 −20 19.5 −30 11.5 −40 9.1 18 3 6 −10 22.6 −2020.8 −30 5 −40 — 19 4 6 −10 25.6 −20 19.6 −30 8.2 −40 2.4 20 5 6 −1025.9 −20 19.3 −30 7.6 −40 4.6 21(c) 7 6 −10 18 −20 17 −30 5.4 −40 2.522(c) 6 6 −10 22 −20 15 −30 5.1 −40 2.1

TABLE 3 IMPACT AMBIENT TEMPERATURE COLD IMPACT ADDITIVE IMPACT (kJ/m²)(kJ/m²) PBT* 69,3 11,3 Example 10 62 16,1 Example 11 56,8 8,4 Example 1274 8,8 *COMPOSITION BASED ON PBT (WITHOUT IMPACT ADDITIVE)

TABLE 4 AMBIENT AMBIENT TEMPERATURE TEMPERATURE IMPACT COLD IMPACT,IMPACT IMPACT ZONE CHARPY CHARPY ADDITIVE (kJ/m²) (kJ/m²) (kJ/m²) PVDF* 7,5 8  3,7 Example 10 62,8 78,2 15,6 Example 11 49,3 60,3 11,1*COMPOSITION BASED ON PVDF (WITHOUT IMPACT ADDITIVE)

From a review of the examples and the specification, it is clear thatthe core is of two types: with a covering composition and without acovering composition. When with a covering composition, said corecontains above zero, preferably at least 5%, more preferably at least10%, by weight of the covering composition, with the preferred maximumpercentage being about 80% by weight.

Also, the preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

The entire disclosure of all applications, patents and publication,cited above and below, and of corresponding French 95/12706, are herebyincorporated by reference.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

In this specification and the following claims, the expression “rangingfrom 7 to 9,” e.g., from 5 to 12 and the like includes values x and y,e.g., 5 and 12. Also, the abbreviation “m.g.” represents meter-gram(s).

We claim:
 1. Impact additive of the core/shell type composed of a corebased on alkyl acrylate or on a polyorganosiloxane rubber and a shellbased on poly(alkyl methacrylate), or on a styrene-acrylonitrilecopolymer, characterized in that the said impact additive comprisesfrom: a) 70% to 90% by weight of a crosslinked elastomeric core which iscomposed: 1) of 20% to less than 100%by weight of a nucleus composed ofa copolymer (I) of n-alkyl acrylate, the alkyl group of which has acarbon number ranging from 5 to 12, of a polyfunctional crosslinkingagent possessing unsaturated groups in its molecule, at least one ofwhich is of a vinyl group, and optionally of a polyfunctional graftingagent possessing unsaturated groups in its molecule, at least one ofwhich is an allyl group, 2) of more than 0 and to 80% by weight, of acovering composed of a copolymer (II) of n-alkyl acrylate, the alkylgroup of which has a carbon number ranging from 4 to 12, or and of agrafting agent possessing allyl groups, the said covering containing amolar amount of grafting agent ranging from 0.05% to 2.5%, said graftingagent having only allyl functional groups, all having the samereactivity, b) 30% to 10% by weight of a shell grafted onto the saidcore composed of a polymer of an alkyl methacrylate, the alkyl group ofwhich has a carbon number ranging from 1 to 4, or alternatively of astatistical copolymer of an alkyl methacrylate, the alkyl group of whichhas a carbon number ranging from 1 to 4, and of an alkyl acrylate, thealkyl group of which has a carbon number ranging from 1 to 8, containinga molar amount of alkyl acrylate ranging from 5% to 40%, oralternatively composed of a styrene-acrylonitrile copolymer.
 2. Anadditive for improving impact resistance, said additive comprising: a)70-90% by weight of a cross linked elastomeric core compound of 1)20-90% by weight of a nucleus comprising a copolymer of n-octyl acrylateand 1,4-butanediol diacrylate, and 2) surrounding said nucleus, 80-90%more than 0 to 80% by weight of a covering comprising a copolymer ofn-octyl acrylate and diallyl maleate, said covering containing a molaramount of diallyl maleate from 0.05% to 2.5 % and b) surrounding saidcore, 30-10% by weight of a shell grafted on to the said core, saidshell composed of a polymer of an alkyl methacrylate, the alkyl group ofwhich has a carbon number ranging from 1 to 4, or alternatively of astatistical copolymer of an alkyl methacrylate, the alkyl group of whichhas a carbon number ranging from 1 to 4, and of an alkyl acrylate, thealkyl group of which has a carbon number ranging from 1 to 8, containinga molar amount of alkyl acrylate ranging from 5% to 40%, oralternatively composed of a styrene-acrylonitrile copolymer.
 3. Anadditive according to claim 2, wherein said nucleus is about 90% byweight of said core and, said covering is about 10% by weight.
 4. Anadditive according to claim 3, wherein said shell consists essentiallyof poly(methyl methacrylate).
 5. An impact resistance additive accordingto claim 2, wherein said impact additive comprises from: a) 75% to 85%of said crosslinked elastomeric core, b) 25% to 15% of said shellgrafted onto the said core.
 6. An impact additive according to claim 2,wherein the covering of the crosslinked core has a molar amount ofgrafting agent of between 0.5% and 1.5%.
 7. An impact additive accordingto claim 2, characterized in that the alkyl methacrylate used to formthe shell is methyl methacrylate.